Photochromism is defined as “a reversible transformation of a single chemical species being induced in one or both directions by absorption of electromagnetic radiation, with two states having different distinguishable absorption spectra.” Thus, photochromic compounds are compounds that possess at least two isomeric forms which have different physical properties, such as absorption properties, refractivity, and the like, and can be transformed from one form to another by light excitations at prescribed wavelengths.
Photochromism has been intensively studied due to its potential use for optical recording and other optical functioning devices. To be practically used as optical recording materials, both isomeric forms must be thermally stable and possess excellent durability for reversible photochromic reactivity. Diarylethene is one class of photochromic compounds which possesses these properties, and therefore is a suitable class of compounds for the construction of optical functioning devices. The cis-configuration of both aryl groups in the diarylethenes studied is generally fixed by an upper cycloalkene structure, such as fluorinated alicyclic group, aromatic group, anhydride and maleimide group. Apart from the difference in absorption characteristics and the like between the two forms and their thermal stabilities, the availability of desirable excitation wavelengths that can be tuned and selected for the photochromic reactions also represents an important aspect in the design of materials for optical functioning devices. It has been shown that with the more n-conjugated upper cycloalkene structures, such as maleimide derivatives, in the diarylethene compounds, the photocyclization proceeded with lower energy excitation in the visible region.
Further information can be found in U.S. Pat. Nos. 5,175,079, 5,183,726, 5,443,940, 5,622,812, and 6,359,150; Japanese patents JP 2-250877, JP 3-014538, JP 3-261762, JP 3-261781, JP 3-271286, JP 4-282378, JP 5-059025, JP 5-222035, JP 5-222036, JP 5-222037, JP 6-199846, JP 10-045732, JP 2000-072768, JP 2000-344693, JP 2001-048875, JP 2002-226477, JP 2002-265468 and JP 2002-293784; and in Irie et al., “Thermally Irreversible Photochromic Systems. Reversible Photocyclization of Diarylethene Derivatives”, Journal of Organic Chemistry, 1988, 53, 803-808, Irie et al., “Thermally Irreversible Photochromic Systems. A Theoretical Study”, Journal of Organic Chemistry, 1988, 53, 6136-6138, and Irie, “Diarylethenes for Memories and Switches”, Chemical Review, 2000, 100, 1685-1716. The photochromic compounds of this invention can be used in the same way as described in these references.